π-Charge distribution in some (4-R1-phenyl)bis(ethylsulfonyl)methane and (4-R1-phenyl)bis(benzylsulfonyl)methane carbanions in acetonitrile

The π-electron densities in the carbanions of (4-R1-phenyl)bis(ethylsulfonyl)methanes and (4-R1-phenyl)bis(benzylsulfonyl)methanes were calculated from 13C NMR shifts in acetonitrile using the method of Bradamante et al. According to this analysis about half of excess π-electron density is shifted from the α-carbon to the two ethylsulfonyl or benzylsulfonyl groups. The charge delocalization over the 4-R1-phenyl group is relatively weak which is consistent with low sensitivity of pKa values of these carbon acids to the para substitution. Nevertheless the small changes in π charge at Cα, caused by substitution on the phenyl, correlate well with the acidity of carbon acids in acetonitrile. The pKa values for nine disulfonyl carbon acids in acetonitrile have been determined by conductometric method; they fall in the range 19-24.